March 19, 2013

Ultrasonic Sensors

I’ve been working on sensors for a bit. One of the things that bothers me is that 3d printers and mills use an open loop system. I’ve found that open loop systems have error inherent in them, perhaps a 1/10K variance. I’ve seen this error in multiple places, and it can sometimes show up on large objects. So, I’ve been thinking of a closed loop system. At the very least, replacing the contact-based microswitches with a positionable hall effect sensor and magnet makes a lot of sense.

So, my previous post was on how to code up and use a hall effect sensor. I wrote it because the various tutorials online clash with each other, and I wanted to make one that matched my own experiments.

I just finished testing an UltraSonic sensor ( the HC-SR04 — about $6 on Amazon. ), and have discovered a few things.

1. At very short distances, there’s a large variance in the output. It’s pretty much blind at some distance less than 1cm.

2. At distances beyond 20mm, the variance drops a lot. Variance is around 4 micro-seconds( uS ).

3. The distance measurements computed from the output are off a bit, depending on variances in the speed of sound at the moment ( temperature, air density, and possible some interference from the surface material ).

These errors make it a very difficult sensor to use for an accurate system like a linear motion platform. However, I believe it may be possible to use it under some circumstances. I may investigate further — I’m not sure.